Amplifiers find many uses in communication systems. For example, high performance amplifiers are integral components of consumer products such as tuners for digital direct broadcast satellite (DBS) applications. In particular, high performance amplifiers are required in direct-conversion tuner products designed for use in DBS and other applications. Direct-conversion architectures reduce the required amount of off-chip componentry and overall system costs as compared to intermediate-frequency (IF) solutions.
Complicating the design of communications systems is the fact that the amplitude of signals reaching a receiver can vary dramatically for a number of reasons, particularly on channels that utilize RF propagation. Satellite signals, for example, can be affected by weather, clouds and rain. Similarly, wireless network signals may be affected by distance and fading.
In this context, automatic gain control (AGC) functionality is often employed to develop a constant power level for use by subsequent stages of the communication system. It is beneficial to provide a known signal level via gain adjustments in order to economically and reliably design processing stages that operate near optimum drive and output levels. Typically, optimal levels are determined by the relationship of signal level to noise or distortion limits. However, AGC circuitry, particularly of wide dynamic range, may have deleterious effects on the noise figure (NF) performance of a receiver. For this and other reasons, low noise amplifiers (LNAs) are needed for many communications applications.
The dynamic range of active components such as LNAs is typically defined on the low-output signal side by the noise figure (NF), and on the high-output signal side by intercept points (e.g., the intercept point second order, IP2, and the intercept point third order IP3). Intercepts points indicate how much output level can be achieved before limitations occur due to undesired distortions. An intercept point is actually a fictitious, extrapolated point on an output versus input curve for a given device. Output level limitations may be manifested as nonlinearities in the response of a device, which in turn may appear as harmonics of an input signal.
The implementation of devices such as a digital satellite (e.g., Direct Broadcast Satellite or DBS) direct conversion tuner integrated circuit that does not require a front-end tracking filter requires a wideband, high dynamic range, variable gain amplifier. In such applications, a low NF figure is required when amplifying a satellite signal with maximum gain. Further, when the satellite signal is received at maximum strength, it must be attenuated (sometimes more than 30 dB over the L-band of 950 MHz to 2150 MHz) while introducing very low distortion. While the gain is reduced from its maximum value to its minimum value, the IP3 is preferably variable from -10 dBm to +12 dBm, while peaking at +18 dBm in between these extremes, in order to meet overall IP3 specifications. The RF front end (including the variable gain amplifier and mixer circuitry) may require -12 dBm IP3 at full gain and +12 dBm at full attenuation. In addition, the foregoing specifications must be met over wide process and temperature variations. It is also desirable to provide on-chip matching to the characteristic impedance of an associated antenna.
Many prior integrated LNA designs suffer from a variety of shortcomings, including discontinuous gain curves and relatively poor linearity and noise performance. The need to compensate for such disadvantages increases overall system implementation costs and complexity.